Fuel burners, and particularly gas fired furnaces, operate under the control of thermostats. The overall system normally includes an ignition source, a fuel valve for pilot and main fuel control, and a temperature responsive switch to prove the existence of a pilot flame prior to introducing the main fuel flow.
In conventional gas fired furnaces, the presence of a pilot flame is proved by a temperature responsive switch that operates in response to the presence or absence of a pilot flame. The time for this switch to operate is a finite period of time as the switch may be a bimetal operated flame responsive member. In order to allow for the system to properly function, the system typically also includes a time delay circuit to allow complete stabilization of the pilot flame after the pilot flame sensing switch has operated. These time delays can be of sufficient length to cause problems in modern day electronic type thermostats. Many of the present day thermostats require a continuous current load when the thermostat is closed (that is, calling for heat). This current loading must take into consideration the various levels of current drawn by the fuel valve during its various stages of operation.